# Density of a planet or moon

##
If we know the mass *M* and radius *R* of a planet or moon,
we can compute its density, which is its mass divided by its volume. Here is the formula for the density of a sphere:

If you divide by the same formula for the density of some standard object
like the earth or our moon, using *M*_{0} and *R*_{0} for the mass and radius of the standard object, you have

Thus we get

This formula is nice and simple.

ASTR 121 Home

Davison E. Soper, Institute of Theoretical Science,
University of Oregon, Eugene OR 97403 USA
soper@bovine.uoregon.edu